The micrometer is an accessory for the microscope that allows measurements to be taken of minute objects. One early method of measuring through a microscope involved placing a fine lattice of silver wires between the microscope's lenses. 'Micrometry' is the technique of using micrometers, though the term was only coined in the 19th century. In the 18th century, however, there was considerable debate about the importance of microscopic measurement, and also how it was to be achieved. Here the debate is illustrated by a pamphlet from the late 18th century.
This statement by Benjamin Martin, though made in a pamphlet (circa 1775) promoting his own design of micrometer, may be taken seriously as his view on the correct use of a microscope. It was Martin who had, in 1732, resolved the problem of convenient measurement with a microscope. The variety of attempts before that date illustrate both the difficulty of and need for an accurate method of assessing the size of microscopic phenomena.
The micrometer was important not only for the measurement of specimens: whilst there is a way to find the magnification power of lenses by measuring their dimensions, this was not practical for most natural philosophers in the 18th century,. The lenses themselves were not made to a high, and regular, enough standard to allow any generalisations to be made. Indeed, Martin lists the primary use of the micrometer as being:
Martin stressed that merely observing Nature's minute curiosities did not pay due respect to their Creator; one had to attain "a knowledge of the amazing Gradations of diminishing Magnitude." Following this, he gives a critique of earlier attempts at micrometry, which amounts to a brief history of the subject:
"it was long after the Invention of the Microscope, before a Word was Mentioned of a Micrometer; Power, Leeuwenhoek, Hooke, Swammerdam, Hugenius, Baker, and many others who have wrote largely on this Subject, are either wholly silent about it, or mention only Grains of Sands, Shreds of fine Wire, Lattices in Brass, &c."
The "grains of sand" were Dutch microscopist Antoni van Leeuwenhoek's point of reference under the microscope. "Shreds of fine wire" was an ingenious method proposed by James Jurin in 1718 that involved winding silver wire around a pin until an inch had been covered, counting the number of coils and thus working out the wire's diameter; small sections would then be cut and scattered about the microscope stage for comparison with the specimen. The "lattice in brass" was developed by John Cuff to accompany his new design of microscope (Image 1).
In criticising Cuff's approach, as well as his patron Henry Baker, Martin was responding both to competition in business and to the lack of recognition he had received in Baker's extremely popular book, The Microscope Made Easy (1742). The book describes one of Martin's methods - engraving divisions on a glass plate in the lens system - but Baker ignores the more successful screw system proposed in 1732, saying merely:
Boris Jardine, 'The earliest measurements of microscopic objects', Explore Whipple Collections, Whipple Museum of the History of Science, University of Cambridge, 2006 [http://www.hps.cam.ac.uk/whipple/explore/microscopes/measuringthemicroscopic/, accessed 22 February 2017]